1. Field of the Invention
The present invention relates to a composition of a solid or liquid coating as an impurity dopant source, and a laser-assisted diffusion of the composition leading to a time- and cost-effective fabrication method of solar cells.
2. Description of the Related Prior Art
The formation of the cell junction in the process of manufacturing silicon solar cells is realized, in part, by the diffusion of different types of impurities or dopants into single crystal and multi- or poly-crystalline silicon wafers. This is achieved by depositing a compound containing the desired dopants onto the cell surface, followed by heating at high temperatures to allow silicon wafers to anneal so that dopant atoms penetrate into the cell surface by thermal treatment.
The effectiveness of the solar cell fabrication based on the above-disclosed traditional thermal diffusion, thus, depends on the duration of the diffusion step and the number and/or size of furnaces providing for simultaneous thermal treatment of multiple wafers. The overall time associated with traditional fabrication methods may be on the order of tens of minutes, which is long for high output production. Furthermore, the simultaneous thermal treatment of a plurality of coated wafers, which is necessary for the efficient cell manufacturing, requires multiple, large diffusion furnaces are costly.
One of the latest trends in the solar cell fabrication includes depositing of dopants by means of laser radiation which is typically generated by pulsed lasers. In general, there are two laser-assisted diffusion techniques: the laser-induced melting of predeposited impurity doping (LIMPID) (See The Dopant Diffusion Mechanism in Excimer Laser Induced Molten Silicon, Toshiyuki et al, Symposium Proceedings of the Materials Research Society, Vol. 101, pp. 491-494—fully incorporated herein by reference.)
The LIMPID technique is associated with a solid layer as a dopant source which is typically deposited by chemical vapor deposition (CVD) or sputtering. Once the deposition of the solid layer is completed, the laser diffusion is performed by exposing the coated wafer to laser radiation. This technique is typically associated with a wafer enclosed in a vacuum chamber to prevent oxidation of the molten Si surface, which is undesirable. The cost of the chambers and vacuum systems may render the discussed technique coast-ineffective.
The gaseous induced laser diffusion (GILD) technique is associated with the uncoated wafer enclosed in a vacuum chamber which contains the dopant gas at reduced pressure minimizing the oxidation of the wafer's surface during subsequent laser irradiation. (See U.S. Pat. No. 5,323,013) The laser radiation enters the chamber through a window to diffuse the dopant from gas phase into the annealed wafer surface forming the semiconductor junction. This technique, like the above discussed LIMPID, may be lengthy for modern mass production methods because many of the methods steps are performed sequentially.
A need, therefore, exists for a cost-effective mass-production fabrication process of semiconductor junctions utilizing Rapid Thermal Annealing (RTA) by laser radiation in the presence of anti-oxidation gases for depositing a suitably uniform coating within about one second or less at an ambient pressure.
A further need exists for an apparatus operative to realize the disclosed method.